Process for the final evaporation of black liquor

ABSTRACT

The process for the final evaporation has at least two evaporation appliances. These appliances are coupled in series with respect to the black liquor and in parallel with the live steam (primary steam), which drives the evaporation, and with the secondary steam that is generated by the evaporation. The pressure of the primary steam that is supplied to the appliance(s) where the liquor concentration is at the highest concentration is greater than the pressure of the primary steam supplied to the other appliances where the liquor concentration is not so high.

TECHNICAL FIELD

The present invention relates to an improved final evaporation of blackliquor from cellulose digesting in several, at least two, evaporationappliances of a type which is known per se.

STATE OF THE ART

Spent liquor from cellulose digesting, so-called black liquor, has a drysubstance content of 14-17%. These substances are digesting chemicalswhich are to be recovered and organic substances, principally ligninresidues, which are to be combusted. As a result of this lowconcentration of solid substances, the black liquor cannot be combusteddirectly and has, therefore, to be concentrated by evaporating offwater. This evaporation takes place in different devices and results ina concentrated black liquor with a concentration in the region of 80%.This concentrated black liquor is then combusted in a recovery boilerwhile recovering chemicals and heat energy.

The evaporation usually takes place in a series of appliances which, inprinciple, are constructed as heat exchangers. They consist of acylindrical container of substantial height, for example 8-10 meters,and are provided internally with heat-exchanging tubes orheat-exchanging plates. In a preferred embodiment, the heat exchangerconsists of a tube assembly which is vertically placed between two tubeplates which are drilled directly in front of the ends of the tubes sothat liquid, black liquor, can be brought to flow vertically downwardsinside the tubes while steam, which is to bring about the evaporation,is allowed to flow and condense on the outside of the tubes. The ends ofthe tubes are joined to the tube plates so that there is nocommunication between the inside and the outside, respectively, of thetubes. The black liquor consequently forms a falling film inside thetubes and gives off some of its water content in the form of steam,which is used for evaporation in other appliances. An appliance of thisnature is termed a falling film evaporator. Normally, several suchappliances are coupled in series. While they can be arranged so that theliquor flows countercurrent to the steam, concurrent devices are alsoknown. In these latter devices, it is usual to have heat exchangerslocated between the evaporation appliances in order to increase thetemperature of the liquor.

At atmospheric pressure, thin liquor having a dry substance content of14-17% boils slightly above the boiling point of water (100° C.).However, the boiling point of the black liquor will increase graduallyas the liquor is concentrated, and the elevation of the boiling pointis, for example, approximately 8° C. for a liquor having a dry substancecontent of approximately 50%, approximately 12° C. for a liquor having adry substance content of approximately 60%, and approximately 25° C. fora very concentrated liquor having a dry substance content ofapproximately 80%. This means that the steam which is fed on the outsideof the tubes, and which is to bring about the evaporation, must have acondensation temperature which exceeds the boiling point of the mostconcentrated liquor, i.e. when the counterpressure of the evaporatedsteam on the liquor side is, for example, 1 bar (excess pressure)corresponding to a condensation temperature of 120° C., a temperature inthe region of 145° C. must be imparted to the liquor film. In thoseappliances with a lower concentration of dry substance, for example 70%and 60%, the liquor film boils at 136 and 132° C., respectively.

It is previously known to feed steam, primary steam, which is at thesame pressure and temperature, to all the appliances involved in thefinal evaporation effect and to bring together the evaporated steam fromall the appliances into one and the same line under the same pressure.In such a case, therefore, the differences in temperature between theprimary steam and the boiling points in the different appliances will bedifferent.

In order to minimize the heat surface and to make it possible to washthe final evaporation effect, the latter is often divided into severalappliances with the spent liquor being conveyed in series through theappliances. Since the same steam pressure pertains in all the appliancesof the final evaporation effect on both the primary and secondary steamsides, the appliance containing the highest dry substance content, andtherefore exhibiting the greatest increase in boiling point, willprincipally determine the lowest possible difference between thesaturation temperatures of the primary and secondary steam sides. Fromthe point of view of heating technology, the difference in temperaturebetween the primary steam and the liquor will be unnecessarily large inthe appliances with lower concentrations.

When the dry substance content is increased, the viscosity alsoincreases. This increase in viscosity is especially pronounced at highdry substance contents (>50%). This increased viscosity impedes heattransfer. Since one and the same appliance operates with a dry substancecontent which corresponds to the outgoing dry substance content, theoutgoing dry substance content will determine the heat transfer in thewhole of this appliance. Dividing the final evaporation effect intoseveral appliances, with the black liquor being conveyed in seriesthrough the appliances, renders it possible for one or more appliancesto operate with a black liquor which is of lower dry substance contentthan that contained in the appliance which has the highest content. Thiswill then facilitate heat transfer. There are, therefore, a number ofgrounds for dividing the final evaporation effect into severalappliances.

European Patent Specification 365 060 describes a process in which theevaporated steam from an evaporation appliance is conveyed onwards to asecond evaporation appliance with a higher dry substance content and inwhich the evaporated steam flows in on the liquor side, thereby shearingthe falling film. This has the effect of improving heat transfer. Thisprocess is often termed steam recirculation.

TECHNICAL PROBLEM

While the temperature difference will therefore be small in theappliance(s) in which the concentration of dry substance in the liquoris highest, it will become ever larger as the concentrations decline inthe upstream part of the system. This is not satisfactory from the pointof view of heat economy, and various attempts have been made to improvethe economy of the evaporation process.

SOLUTION

In accordance with the present invention, a process has therefore beenproduced for the final evaporation of black liquor in at least twoevaporation appliances which are of a construction which is known per seand which are coupled in series with respect to the black liquor and inparallel with respect to the live steam (primary steam) which drives theevaporation, with the steam (secondary steam) which is generated by theevaporation being in open communication in both or all of theappliances, which process is characterized in that the pressure of theprimary steam which is supplied to the appliance(s) in which the liquoris at the highest concentration is greater than that at which theprimary steam is supplied to the other appliance(s).

According to the invention, it is expedient for the evaporation to beundertaken in at least two, preferably three or four, appliances ofwhich the last, seen in the liquor direction, is fed with primary steamwhich is at a higher pressure than the steam fed to the remaining three.If three or more appliances are being used, it can be advantageous alsoto increase the steam pressure in the penultimate appliance, seen in theliquor direction.

In a mill for producing cellulose pulp, there are steam nets which areat different steam pressure levels. The so-called low pressure level(3-4 bar (excess pressure)) is normally used to drive the evaporation.For example, the primary steam to the first effect can then be at apressure of 3 bar (excess pressure), corresponding to a condensationtemperature of 143° C. The first effect often constitutes the finalevaporation effect. According to the invention, one or more appliancesare fed with primary steam at a pressure of, for example, 3 bar (excesspressure) while the primary steam pressure for the other appliances,which are fed with primary steam at a higher pressure, is, for example,4.4 bar (excess pressure) (a condensation temperature of approximately155° C.). This steam pressure is selected so that the resultingdifference in temperature between the steam side and the liquor side isapproximately equal to, or preferably somewhat greater, than that whichpertains in the appliances for which the primary steam pressure islower.

According to the invention, the pressure of the secondary steam can be0.7 bar (excess pressure) and its saturation temperature can be greaterthan 115° C.

As a result of the process according to the present invention, the drysubstance content of the liquor will exceed 70% when the latter leavesthe final appliance, seen in the liquor direction.

The process according to the invention preferably comprises a so-calledfalling film process.

According to the invention, it is expedient for evaporated steam fromappliances in the final evaporation effect which contain liquor of lowerconcentration to be supplied to the appliances which contain liquor ofhigher concentration together with the liquor for shearing the fallingfilm. That is, the above described process, the so-called steamrecirculation process is applied.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail below with reference tothe attached drawings in which

FIG. 1 is a representation, partly in section, of an evaporationappliance which is used in accordance with the present invention,

FIG. 2 is a more diagrammatic representation of an evaporation plant inaccordance with the present invention, and

FIG. 3 is a diagrammatic representation of the process according to theinvention.

DETAILED DESCRIPTION

FIG. 1 shows an evaporation appliance which is used in the processaccording to the present invention. This appliance consists of avertically elongated, virtually cylindrical outer casing 1 whose lowerpart is somewhat widened. Inside the cylindrical casing 1, there is aheat exchanger 2, which preferably consists of tubes through which theliquor is intended to run downwards and around which the primary steamis intended to condense in order to heat the tubes and evaporate theliquor. The tube assembly 2 is joined by its tube ends to a tube plate 3at its upper part and to a tube plate 3 at its lower part. The tubeplates 3 are drilled so that the tube ends coincide with these drillholes. A spraying device 4, for example, is present in the upper partfor supplying liquor, which spraying device 4 distributes the liquoruniformly over the tube plate 3 and down through the tubes. The primarysteam, which is intended to flow around the tubes 2, is introducedthrough an opening 5. Condensed steam is removed at the opening 6, andthe concentrated liquor is removed at the opening 7 at the bottom of theappliance. Arrangements are made for returning a part of theconcentrated liquor from the opening 7 to the spraying nozzle 4 in theupper part. Liquor which is to be concentrated is normally supplied inthis return system, i.e. through one of the tube openings in the lowerpart of the appliance.

The appliance shown in FIG. 1 is preferred for the process according tothe invention. It is constructed according to the falling film principleand is highly efficient. Flat heat-transferring surfaces can be usedinstead of tubes, and, in accordance with the invention, it is alsopossible to conceive of a process in which the liquor rises, a so-calledrising film technique.

FIG. 2 shows five so-called effects for the evaporation, with effect Iconsisting, in the drawing, of four units in accordance with FIG. 1, andeffects II, III, IV and V consisting of individual units according toFIG. 1. Heat exchangers 8 of a suitable type are arranged between theseeffects for the purpose of raising the temperature of the liquor.

The final evaporation effect, namely effect I, is fed with live steam 9,with the four units being fed in parallel. As a result, it is easy toclose down any one of them for cleaning. Liquor which has previouslybeen concentrated in effects II, III, IV and V is fed into effect I at10, and concentrated, finally evaporated liquor is removed at 11. A partof the live steam 9 is conducted to the heat exchanger 8 for the purposeof heating the liquor 10 which is to be fed into effect I. Thecondensate from this heat exchanger 8, and condensate from effect I, isconducted out at 12. Black liquor which is to be evaporated is conductedinto effect III. Thin liquor from effect III is fed into effect IVthrough line 14, and from IV to V through line 15. Steam for evaporatingthe liquor in the different effects is taken from the preceding effectthrough lines 16, and heating steam for the heat exchangers 8 is takenfrom the preceding effect through lines 17. Condensate from thedifferent units is removed through tubes 18. A heat exchanger 19 isarranged for final condensation of the evaporated steam from the lasteffect V. The device as shown in FIG. 2 is only one example of deviceswhich can be used for the process according to the present invention.Thus, it is both possible and normal, for example, to arrange theevaporation in effects II, III, IV and V in accordance with thecountercurrent principle. There is then no requirement for heatexchangers between the effects.

FIG. 3 is a diagrammatic representation of the process according to theinvention. The units IA, IB and IC are units in the final evaporationeffect, that is three units in effect I as depicted in FIG. 2, each ofwhich units is as shown in FIG. 1. According to former principles, livesteam which was at the same pressure and temperature was introduced intoall three of the units IA, IB and IC. FIG. 2, for example, depicts livesteam being supplied in this way. However, in accordance with thepresent invention, live steam is supplied at a higher pressure to unitIA than to units IB and IC, namely, for example, 4.4 bar (excesspressure), and at a temperature of 155° C., as compared with 3.0 bar(excess pressure) and a temperature of 143° C. For example, the liquorwhich is fed into unit IC has a dry substance content of 51%, with theoutgoing dry matter content being 63% and the boiling point elevation inIC therefore being approximately 12.4° C.; the dry substanceconcentration of the liquor in IB is 71%, and the boiling pointelevation in IB is 16.1° C.; by contrast, the dry substanceconcentration in the liquor entering IA is 80%, which corresponds to aboiling point elevation of 24° C. The liquor which is removed from IA ishighly concentrated liquor, i.e. liquor which has a dry substancecontent of 80%.

The units IA, IB and IC are in open communication with each other asregards their production of secondary steam 20, which is adjusted to acounterpressure of approximately 1.0 bar (excess pressure), whichdenotes a condensation temperature of 120° C. This steam is conveyedonwards to effect II.

According to the invention, the temperature difference between theoutside and the inside of the tubes in unit IA is approximately 11° C.,namely 155° C. on the outside and 144° C. on the inside (120° C. (at asteam pressure of 1.0 bar (excess pressure)) plus the boiling pointelevation of 23.9° C.). This results in efficient evaporation whichexceeds by far that which it would be possible to achieve using theprocess in accordance with the previously known technique, when steam atapproximately 3 bar (excess pressure) was supplied to all the units IA,IB and IC. If, instead of passing steam at different pressures intothese units, the pressure was raised and steam which was at the samepressure was passed into all the three units, this would then give riseto an unnecessarily large temperature difference in units IB and, inparticular, IC.

As a result of the process according to the invention, which results inunit IA being much more efficient, this latter unit can be made smaller,with lower investment costs as a consequence, and the higher pressurewhich is possible to produce in the secondary steam can also beexploited for, where appropriate, introducing an additional effect orfor decreasing the heat surface in the remaining effects, since agreater temperature difference is available.

When the above example is implemented, the heat surface which isrequired in effect I, comprising units IA, IB and IC, becomes 15% less.The higher pressure of the secondary steam can, in turn, be exploitedfor decreasing the heat surface in the remaining effects by 15%. In thisway, the total heat surface in the plant becomes 15% less. The quantityof live steam at higher pressure which has to be supplied to unit IA isonly 25% of the total quantity of live steam.

As has been mentioned above, low pressure steam (3-4 bar (excesspressure)) is normally used for driving the evaporation. This obliges usto use steam at higher pressure. Normally, medium pressure steam is alsoavailable in a pulp mill, which steam is usually at a pressure which isgreater than 10 bar (excess pressure). This pressure is unnecessarilyhigh in the present context and it may be necessary, therefore, toreduce the pressure of this steam. Increased use of medium pressuresteam will lead to less steam passing through the turbine and, as aconsequence, less electricity being generated. Instead of reducing thepressure of the medium pressure steam, by means of a reducing valve, themedium pressure steam can be used to drive a steam ejector which sucksin low pressure steam and raises the pressure of this steam. A processof this nature should then be able to decrease the requirement formedium pressure steam and would represent a method of compressing thelow pressure steam by mechanical means.

The invention is not limited to the example described above and,instead, can be varied in different ways within the scope of the patentclaims.

We claim:
 1. A process for a final evaporation of black liquor, comprising the steps of:supplying a black liquor to a first evaporation appliance to a second and a third evaporation appliance in series, the first evaporation appliance having a first concentration of solids and the second evaporation appliance having a second concentration of solids, the first concentration being greater than the second concentration; connecting a primary steam carrying member of the second evaporation appliance to the third evaporation appliance in parallel; connecting a secondary steam carrying member of the first evaporation appliance to the second evaporation appliance so that the first and second evaporation appliances are in open fluid communication; supplying a first primary steam to the first evaporation appliance at a first maximum pressure and a first temperature; supplying a second primary steam to the second evaporation appliance at a second maximum pressure and a second temperature, the first maximum pressure being greater than the second maximum pressure and the first temperature being greater than the second temperature; driving an evaporation process with the first and second primary steam; and generating a first secondary steam by the evaporation process in the first evaporation appliance and generating a second secondary steam by the evaporation process in the second evaporation appliance, the first secondary steam having a first secondary steam pressure and the second secondary steam having a second secondary steam pressure, the first secondary steam pressure being identical to the second secondary steam pressure so that a resulting difference in temperature between a steam side and a liquor side of the first evaporation appliance is equal to or greater than a temperature difference between a steam side and a liquor side of the second evaporation appliance.
 2. The process according to claim 1 wherein the process comprises passing black liquor through the second evaporation appliance and then passing the black liquor through the first evaporation appliance and feeding the first primary steam to the first evaporation appliance and feeding the second primary steam to the second evaporation appliance.
 3. The process according to claim 2 wherein the process further comprises the steps of providing a pulp mill containing a low pressure steam having a pressure, the first maximum pressure of the first primary steam being greater than the pressure of the low pressure steam.
 4. The process according to claim 1 wherein the process further comprises the step of concentrating the black liquor supplied to the first evaporation appliance so that a dry substance content of the black liquor exceeds 70% when the black liquor leaves the first evaporation appliance.
 5. The process according to claim 1 wherein the process further comprises the step of evaporating the black liquor according to a falling film technique by permitting the black liquor to fall downwardly through the first and second evaporation appliances.
 6. The process according to claim 5 wherein the process further comprises the steps of providing a heat exchanger having tubes disposed therein for shearing the falling film disposed on an inside of the tubes.
 7. The process according to claim 1 wherein the process further comprises the step of increasing the steam pressure of the first and second primary steam by providing a steam ejector containing a driving steam having a pressure that is greater than the pressures of the first and second primary steam.
 8. The process according to claim 1 wherein the method further comprises increasing the pressure of the first and second primary steam with a mechanical compressor.
 9. A process for a final evaporation of black liquor, comprising the steps of:coupling a black liquor carrying member of a first evaporation appliance to a second and a third evaporation appliance in series; connecting a primary steam carrying member of the second evaporation appliance to the third evaporation appliance in parallel; connecting a secondary steam carrying member of the first evaporation appliance to the second evaporation appliance so that the first and second evaporation appliances are in open fluid communication; supplying a first primary steam to the first evaporation appliance at a first maximum pressure and a first temperature; supplying a second primary steam to the second evaporation appliance at a second maximum pressure and a second temperature, the first maximum pressure being greater than the second maximum pressure and the first temperature being greater than the second temperature; supplying a pulp mill in operative engagement with the second evaporation appliance, the pulp mill containing a low pressure steam having a pressure, the first maximum pressure of the first primary steam being greater than the pressure of the low pressure steam; driving an evaporation process with the first and second primary steam; generating a first secondary steam by the evaporation process in the first evaporation appliance and generating a second secondary steam by the evaporation process in the second evaporation appliance, the first secondary steam having a first secondary steam pressure and the second secondary steam having a second secondary steam pressure, the first secondary steam pressure being identical to the second secondary steam pressure; passing black liquor through the second evaporation appliance and then passing the black liquor through the first evaporation appliance and feeding the first primary steam to the first evaporation appliance and feeding the second primary steam to the second evaporation appliance; andsetting the low pressure steam to about four bar.
 10. The process according to claim 9 wherein the method further comprises setting the pressure of the secondary steam to a value that is greater than 0.7 bar and setting a condensation temperature of the secondary steam that is greater than 115 Celsius. 